There's something racy here; in my Atomic Workstation CI/CD I'm seeing:
```
01:12:43 symlink /lib/systemd/system/rngd.service etc/systemd/system/basic.target.wants/rngd.service
01:12:43 FileNotFoundError: [Errno 2] No such file or directory: '/lib/systemd/system/rngd.service' -> '/var/tmp/lorax.7cgdtz1_/installtree/etc/systemd/system/basic.target.wants/rngd.service'
```
Rather than debug this right now, let's just make sure it exists,
like we do right above for `tmp.mount`.
